1. Field of the Invention
This invention relates to diazotype and more particularly relates to a method for diazotype on paper, even alkaline paper.
2. Brief Description of Related Art
Diazotypy is a copying process for making copies from originals by exposing them in contact with a diazotype sensitive copying sheet to ultraviolet light and by developing the diazotype sheet thereafter with ammonia or a liquid developer. The diazotype process because of its ease and economics has found wide application during the last 40 years.
Diazotype copying materials are made by applying a light-sensitive diazo coating to the surface of a base, such as foils, opaque paper, translucent paper, or cloth. The active components of the light-sensitive layer are monomolecular, crystalline, not self-supporting and need a resinous or a fibrous substrate as carrier material.
If paper is used as the carrier material, the diazotype coating penetrates into and disperses in the top surface of the paper and the diazotype print lines of the final copy are well anchored within the fibrous structure of the paper.
Base paper for diazotype coatings has previously been acidic to minimize decomposition of the diazo compound as well as precoupling of the diazotype paper upon shelf aging (R. H. Mosher, Specialty Papers page 232, Remsen Press 1950; Douglas Poundrier, Tappi April 1963, Diazo For Papermakers). The pH of diazotype base paper, in general, has previously ranged from 4.5 to 5.
Alkaline sized paper has become popular since the early 1980s for various reasons, such as elimination of equipment corrosion problems from acidic sizing, facilitation of more environment friendly waste water disposal, and using low cost calcium carbonate as filler, which is not compatible with acid sized paper, but can be used for sheet opacity and sheet brightness improvement.
Once paper mills switch from acid to alkaline sizing they cannot practically revert to acid sizing. As a result, the availability of acid sized base paper for the diazo coating industry diminished substantially. Alkaline sized paper, particularly with calcium carbonate as filler, cannot be used for stable conventional diazotype coatings. The alkaline sized paper has poor hold out for the sensitizing solutions and the stabilizing acid from the diazotype coating preparation penetrates into the base sheet. The stabilizing acid is neutralized by a calcium carbonate filler. Precoupling and decomposition of the diazo compounds will occur. The diazotype process is described by Jaromir Kosar: Light Sensitive Systems, John Wiley & Sons, New York.
Diazotypes, in their early beginnings, were made by coating diazo compounds, with or without coupling components, and their stabilizing agents directly on paper base. In the latter 1940's precoating of base paper for diazotypes was introduced. This was carried out by applying a layer of silica or resin dispersions or both to the paper base prior to sensitizing with the light sensitive coating. The objective of this earlier precoating was an optical activation of the print dye appearance for increased print contrast and print color value. The earlier precoating process per se however did not stop the diazotype chemicals from entering into the surface strata of the paper base.
In the 1970s, diazotype intermediates, on transparentized paper, with erasability features were introduced for easy correction of print lines. Erasability was achieved by resin interlayers between the translucent base and the diazotype coating (cf. U.S. Pat. Nos. 3,923,518 and 4,058,399). For easy mechanical erasability it was important that the resin interlayer exhibited only a limited adhesion to the base paper.
Diazotypes for moist development with a controlled minimum amount of liquid developer application to the sensitized side of the paper only, for rapid print drying, were disclosed in U.S. Pat. No. 4,128,423. A resin layer was applied to conventional opaque diazo base paper which prevents wet curl of the prints during the moist development process.
All heretofore practiced diazotype paper used acid sized base paper as discussed above. There is no prior art known to us for the use of base paper with neutral or alkaline sizing or with calcium carbonate as a filler.